The heating of a medical fluid by means of a medical heat exchanger can be accomplished by electric resistance heating elements such as heating plates or by means of electric emitters for light which heat the medical heat exchanger indirectly from the outside.
The document WO 03/061740 A1 describes a medical heat exchanger which is embodied as a disposable heating cassette, such that the heating of the heating cassette takes place from the outside on the one hand by means of an electric light emitter which is coupled to the heating cassette and on the other hand additionally with an electric resistance heating plate coupled to the heating cassette on the opposite side of the heating cassette. In an alternative embodiment, the electric heating plate also has, in addition to the heating cassette, an absorber for infrared radiation which additionally heats the heating cassette from the outside.
The generic medical heat exchangers known from the prior art have considerable disadvantages because the coupling surface of the light emitter on the machine side and the coupling surface of the disposable heating cassette are subject to high thermal loads so that the maximum temperatures that occur must not exceed 80° C., for example, for safety reasons as well as factors pertaining to the materials. Therefore this requires at least large coupling surfaces and a mechanically complex coupling of the disposable heating cassette to the coupling surface of the light emitter on the machine side. Furthermore, suction removal of the air trapped between the coupling surface of the disposable heating cassette and the coupling surface of the light emitter on the machine side may be necessary during coupling. If the coupling surface of the disposable heating cassette is designed as a film, then a complex pressing of the film to the coupling surface of light emitter may be necessary in combination with the suction removal of air. Despite such complex measures, a substantial portion of the radiation of the light emitter must nevertheless be dissipated through component cooling on the machine side and is therefore not available for heating the medical fluid.
When light passes through a material, some of the light is converted into heat and some of the light is allowed to pass through.
Within a very small layer thickness dx of the material, the radiation intensity of the light is reduced by the same fraction, which is described by the absorption coefficients. The absorption coefficient is a substance-specific function, which depends on the wavelength λ of the light; this is referred to as the absorption spectrum μ(λ) of the substance. The higher the absorption coefficient at a certain wavelength, the greater is the absorbed radiation intensity of light of a certain wavelength, which is converted into heat in the very small layer dx.
Similarly, the transmission coefficient describes the passage of light through a very small layer thickness dx of the material. The transmission coefficient is similarly a specific function of the wavelength λ of the light which is referred to as the transmission spectrum μ(λ) of the substance. The greater the transmission coefficient at a certain wavelength, the greater is the radiation intensity of the light of a certain wavelength passing through the very small layer dx.
The absorbance α of a body is defined as the quotient of the absorbed radiation energy and the radiation energy that occurs. The absorbance is therefore a measure for characterizing a body with regard to its property of converting incident light into heat.
The transmittance τ of a body is defined as the quotient of the radiation energy allowed to pass through and the resulting radiation energy.
Bodies which absorb all the radiation energy and convert it completely into heat have an absorbance of 1 and a transmittance of 0 (“absolute black bodies”).
The reflectance ρ of a body is defined as the quotient of the reflected radiant energy and the incoming radiant energy.
It holds that: transmittance τ+absorbance α+reflectance ρ=1 Bodies that reflect all of the incident radiation or allow all of the incident radiation to pass through have an absorbance of 0.
A known advantage of the generic medical heat exchangers heated by a light emitter is the good electric safety because of the spatial distance between the light emitter and the fluid.
A known problem of the generic medical heat exchangers which are heated by a light emitter is the at least local occurrence of inadmissible temperature peaks; this problem occurs in particular with medical heat exchangers made of materials having a low thermal conductivity. As a result of the low thermal conductivity, such medical heat exchangers deliver a substantial portion of the incident heat to the air surrounding the medical heat exchanger, which has a negative effect on the efficiency in heat transfer.
It is known that medical heat exchangers can be produced as disposable heating cassettes (disposable items). For cost reasons, the most inexpensive possible plastics are used for this purpose but they usually have the disadvantage described above of a low thermal conductivity.
Local temperature peaks on the medical heat exchanger may, on the one hand, damage the materials of the medical heat exchanger or may even destroy them due to heating, melting or fire. On the other hand, such local temperature peaks can also damage the medical fluid, but this must be prevented from the standpoint of patient safety in particular. Not least of all, the light emitter may also overheat.
Known structural measures to prevent the at least local occurrence of inadmissible temperature peaks is the use of large heat exchanger surface areas as well as the targeted deflection of radiation and radiation shielding, each of which entails high costs and a reduced efficiency in heat transfer.
One object of the present invention is therefore to provide an improved medical heat exchanger.